Method for operating a vehicle drive device with device with a continuously variable transmission (cvt) in a brake-engaging mode

ABSTRACT

A motor vehicle drive arrangement including a motor and a stepless automatic transmission for driving drive wheels has a normal operating mode for normal driving operation and a brake engagement operating mode for a situation of excessive slip of a drive wheel. During the brake engagement operating mode, strong fluctuations of the transmission ratio could arise, or the vehicle might drive with a low motor torque but a high drive rotational speed, which are uncomfortable for the driver. To increase driving comfort, a normal motor torque value is prescribed in the normal operating mode, and in the brake engagement operating mode the motor torque is reduced to an actual motor torque value. A difference torque is determined between the normal and actual motor torque values. A maximum desired nominal drive rotational speed is varied dependent on the difference torque.

FIELD OF THE INVENTION

The invention relates to a method for operating a drive arrangement of amotor vehicle.

BACKGROUND INFORMATION

The drive arrangements of motor vehicles typically comprise a motor anda transmission, by which the motor torque is transmitted to the drivewheels. Additionally, it is typical to provide brake control devices forspecial braking functions. Such braking functions are known, forexample, by the names or terms: antislip regulation (ASR), electronicdifferential locking (EDS), and electronic stability program (ESP). Thespecial braking functions are active when one or more of the wheels ofthe motor vehicle comprise a slip that exceeds a certain or determinedvalue.

In a motor vehicle with a stepless automatic transmission, thetransmission ratio of the automatic transmission is variable in astepless manner and thus makes it possible to achieve the optimumutilization of the motor torque. For this purpose, the transmissionratio is regulated with a transmission controller or regulator, wherebythe regulation can be carried out according to certain predefinedstrategies. If an increased slip now arises on one of the drive wheels,then this can lead to an increase of the regulation deviation in thetransmission controller. The transmission controller would attempt toregulate-out this regulation deviation, which would lead to rapidchanges of the transmission ratio and therewith to rapid changes of themoments or torques of the drive wheels. Rapid changes of the moments ortorques of the drive wheels would, however, impair the effectiveness ofthe braking functions and lead to a repeated activation and deactivationof the braking functions. In order to avoid this disadvantage, a changeor variation of the transmission ratio of the automatic transmission isprevented, as long as one of the braking functions is active.

This has as a result, that the motor torque will be sharply reduced bythe antislip regulation for a long continuing period, but the motorrotational speed will continue to remain at a high level, during alonger drive on a driving path or roadway with a low coefficient offriction, for example a roadway covered with snow or ice, especially fora drive with a high driving or accelerator pedal angle, i.e. with a highacceleration desire of the driver. This leads to a bothersome high motornoise.

SUMMARY OF THE INVENTION

It is the basic underlying object of the invention to provide a methodfor operating a drive arrangement of a motor vehicle, which makespossible an improvement of the driving comfort with effective brakingfunctions.

The object is achieved according to the invention in a method ofoperating a drive arrangement of a motor vehicle.

In the inventive method, the drive wheels of a motor vehicle are drivenvia a stepless automatic transmission by a motor, whereby a normaloperating mode is prescribed for the normal driving operation and abrake engagement operating mode is prescribed for the case that a slipof at least one of the drive wheels exceeding a certain value isdetected. In the normal operating mode, a normal motor torque value isprescribed as the desired nominal value of the motor torque of themotor. In the brake engagement operating mode, the motor torque isreduced to an actual motor torque value that is smaller relative to thenormal motor torque value, by a motor engagement or interventioneffectuated by a braking function. Furthermore, in the brake engagementoperating mode, the difference between the normal motor torque value andthe actual motor torque value is determined as the difference torque,and a maximum desired nominal drive rotational speed for the driverotational speed of the automatic transmission is prescribed dependenton the difference torque. In this context, the drive rotational speed isunderstood to refer to the motorside rotational speed of the automatictransmission, which is equal to the motor rotational speed with a closedclutch.

Preferably, the maximum desired nominal drive rotational speed in thebrake engagement operating mode is prescribed in such a manner that itis continuously reduced in connection with a value of the differencetorque that exceeds a first threshold value, whereby the reductionadvantageously is carried out with a prescribable speed of change orvariation, and whereby preferably a minimum drive rotational speeddependent on the driving speed, as a lower limit of the maximum desirednominal drive rotational speed, is not fallen below.

In an advantageous further development of the method, a second thresholdvalue lying below the first threshold value is defined as a furtherthreshold value, and, in connection with a value of the differencetorque that falls below the second threshold value, the maximum desirednominal drive rotational speed of the automatic transmission varies insuch a manner that it is increased continuously up to a certain boundaryor limit value, whereby this increase is preferably carried out with aspeed that is dependent on the variation or change speed of thedifference torque.

Advantageously, the maximum desired nominal drive rotational speed isfixedly maintained at a constant value in connection with a differencetorque lying between the first and second threshold values.

The inventive method comprises the following advantages:

-   -   it ensures a low fuel consumption during longer drives with        active braking functions, because the motor is not operated with        unnecessarily high drive rotational speeds;    -   the driving comfort is improved by the reduction of the motor        noises, because the motor is not operated with a low motor        torque and nonetheless a high drive rotational speed;    -   a better support of the braking functions is ensured, because        these are not repeatedly activated and deactivated, and a torque        reserve adequate for the regulation is made available to these        braking functions.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in further detail inconnection with an example embodiment and the Figures. It is shown by:

FIG. 1 a time diagram of the motor torques,

FIG. 2 a time diagram of the drive rotational speeds.

DETAILED DESCRIPTION OF A PREFERRED EXAMPLE EMBODIMENT AND OF THE BESTMODE OF THE INVENTION

The inventive method is utilized for operating the drive arrangement ofa motor vehicle, which comprises a motor as well as a stepless automatictransmission, that is to say a transmission with a stepless variabletransmission ratio. In the control of the drive arrangement, adistinction is made between a normal operating mode and a brakeengagement operating mode, whereby the normal operating mode is providedfor the normal driving operation and the brake engagement operating modeis provided for the case that a slip exceeding a certain value arises onone of the drive wheels of the motor vehicle, which leads to theactivation of a braking function. Next, the control of the drivearrangement will be described in further detail for a certain drivingsituation.

In connection with the curve Mn, FIG. 1 shows the course or progressionof a motor torque value that is designated as a normal motor torquevalue in the following, and that one would obtain as the desired nominalvalue of the motor torque or moment M in the normal operating mode forthe certain driving situation. In this context, this refers to a valuethat is determined dependent on the driving or accelerator pedal angleor throttle flap angle that prescribes the drivers wishes, and dependenton various operating parameters of a motor control device, for examplein connection with a characteristic value field or performance graph.

At the time point t1, a high slip arises on one of the drive wheels, andthe antislip regulation becomes active as a braking function. Thereby,the wheel rotational speed and therewith also the wheel torque on thedrive wheels is controlled by a brake control device. With theactivation of the braking function, the drive arrangement is alsoswitched to the brake engagement operating mode, and the motor torque Mis reduced to an actual motor torque value Ma by the braking functionacting on the motor control device.

During the brake engagement operating mode, the difference between thenormal motor torque value Mn and the actual motor torque value Ma isdetermined as the difference torque Md, and the maximum desired nominaldrive rotational speed is varied dependent on the difference torque Mdin such a manner, that the difference torque Md does not rise too high,so that there will nonetheless still be an adequate torque reserveremaining for the control or regulation carried out by the brakingfunction, without giving rise to the danger that the braking functionwill be repeatedly activated and deactivated within a short time.

In the brake engagement operating mode, the wheel rotational speed ofthe drive wheels is determined by the braking function, so that thedrive rotational speed is controlled by variation of the transmissionratio of the automatic transmission. Thereby, the motor torque M isadjusted to the desired value by the motor control device.

In FIG. 2, the curve nn illustrates the course or progression of themaximum normal desired nominal drive rotational speed, which one obtainsas the maximum value of the desired nominal drive rotational speed n inthe normal operating mode, and the curve na illustrates the course orprogression of the maximum desired nominal drive rotational speed in thebrake engagement operating mode.

According to FIG. 2, the maximum desired nominal drive rotational speedna in the brake engagement operating mode is reduced as soon as thedifference torque Md has exceeded a first threshold value M1, namely atthe time point t2. In this context, the reduction is carried out with acertain variation or change speed and continues until the differencetorque Md again falls below the first threshold value M1. This occurs atthe time point t3. Thereby, a minimum drive rotational speed n0, whichis dependent on the driving speed, is prescribed as the lower limit ofthe maximum desired nominal drive rotational speed na. The prescribingor specifying of this minimum drive rotational speed n0 is carried outin such a manner that for the current driving speed, no unexpectedlyhigh drive rotational speed changes will arise or unexpectedly low driverotational speeds will occur for the driver.

Beginning from the time point t3, the maximum desired nominal driverotational speed na is held constant until the time point t4, namely solong until the difference torque Md falls below a second threshold valueM2 lying below the first threshold value M1. This second threshold valueM2 is selected in such a manner that the torque reserve corresponding tothe difference torque Md in the time interval t2 to t4 is sufficientlylarge, to ensure a reliable performance of the braking function, so thatthe braking function is not repeatedly activated and deactivated withina short time, which would lead to an unsteady rotational speed behavior.The first threshold value M1 is then selected in such a manner so that ahysteresis-formed operating behavior results from the spacing betweenthe two threshold values M1, M2, so that the maximum desired nominaldrive rotational speed Ma does not constantly change or vary withrespect to its value.

Beginning at the time point t4, the maximum desired nominal driverotational speed na increases with a prescribable increasing speed, upto the maximum normal desired nominal drive rotational speed nn as alimit value.

This increasing speed is specified or prescribed dependent on the timechange or variation of the difference torque Md in such a manner so thatthe maximum desired nominal drive rotational speed na increases with anincreased rise or increasing speed, when the difference torque Md risesor increases very quickly, for example due to a sudden shock-likeincrease of the coefficient of friction of the roadway or driving path.Thereby, upon the deactivation of the braking function, that is to sayupon switching over into the normal operating mode, it is ensured thatthe maximum desired nominal drive rotational speed na is equal to themaximum normal desired nominal drive rotational speed nn or is at leastin the proximity thereof. For example, a threshold for the change orvariation speed of the difference torque Md can be defined, and it canbe tested whether the change or variation speed of the difference torqueMd is larger or smaller than the defined threshold. The rise or increasespeed of the maximum desired nominal drive rotational speed na may thenbe switched-over between two values depending on the comparison result,whereby these values are distinguished from one another by the factor of2, for example, whereby the switching-over is carried out in such amanner that a rapid change or variation of the difference torque Md alsohas a rapid change of the maximum desired nominal drive rotational speedna as a result.

1. Method for operating a drive arrangement of a motor vehicle, in whichdrive wheels of the motor vehicle are driven by a motor via a steplessautomatic transmission, comprising prescribing a normal operating modefor a normal driving operation, prescribing a brake engagement operatingmode for a case in which a slip exceeding a certain value is detected onat least one of the drive wheels, prescribing a normal motor torquevalue (Mn) as a desired nominal value of a motor torque (M) of the motorin the normal operating mode, and in the brake engagement operatingmode, reducing the motor torque (M) relative to the normal motor torquevalue (Mn) to an actual motor torque value (Ma), determining adifference between the normal motor torque value (Mn) and the actualmotor torque value (Ma) as a difference torque (Md), and prescribing amaximum desired nominal drive rotational speed (na) for a driverotational speed (n) of the automatic transmission dependent on thedifference torque (Md).
 2. Method according to claim 1, characterized inthat the maximum desired nominal drive rotational speed (na) is reducedin the brake engagement operating mode, in connection with a value ofthe difference torque (Md) exceeding a first threshold value (M1). 3.Method according to claim 2, characterized in that the reduction of themaximum desired nominal drive rotational speed (na) is carried out witha prescribable change speed.
 4. Method according to claim 2,characterized in that a second threshold value (M2) lying below thefirst threshold value (M1) is defined, and in that the maximum desirednominal drive rotational speed (na) is increased in connection with avalue of the difference torque (Md) falling below the second thresholdvalue (M2).
 5. Method according to claim 4, characterized in that theincrease of the maximum desired nominal drive rotational speed (na) iscarried out with an increase speed that is dependent on a change speedof the difference torque (Md).
 6. Method according to claim 4,characterized in that the maximum desired nominal drive rotational speed(na) is held constant in connection with a value of the differencetorque (Md) lying between the first and second threshold values (M1,M2).
 7. Method according to claim 1, characterized in that a minimumdrive rotational speed (n0) dependent on a driving speed of the motorvehicle is prescribed as a lower limit for the maximum desired nominaldrive rotational speed (na).